Numerous processes are known for forming on metal substrates protective coatings that will improve the corrosion resistance of the substrate and/or improve the adhesion of a subsequently applied paint coating. The great majority of these processes are designed for operation under controlled factory conditions. For instance it is well known to form conversion coatings by spraying or dipping with acidic zinc phosphate solutions. Various other phosphating solutions are known as are various chromating solutions.
Many of these processes involve chemical reaction during, for instance, immersion in the treatment solution and as a result satisfactory coating formation occurs almost irrespective of the shape of the surface. However in some processes the coating formation occurs as a result of the treatment solution drying onto the surface, and the shape of the surface can then affect the results, highly convex areas often being inadequately coated. For instance the chromating treatment described in British Patent Specification No. 1,234,181 involves drying the treatment solution onto the surface being coated and although the process has proved very satisfactory for relatively flat surfaces (for instance continuous steel strip) it has not proved very satisfactory for highly curved areas.
Particular problems arise when a protective coating is to be formed on large external structures such as bridge railings and street lights. One difficulty arises from the fact that the treatment solution is likely to drain from highly convex parts of the structure and so an inadequate coating will occur in these areas. Another problem is that the surfaces of the structure are unlikely to have been cleaned to the extent that is conventional in factory processes and it may therefore be difficult to achieve uniform coating of the treatment solution onto the surfaces.
In practice very few systems are available for forming protective coatings on external structures. Frequently no protective coating is applied or, in the case of galvanized iron, the galvanized coating is allowed to age in the atmosphere to form a base for a subsequently applied paint coating. Etch primers based on polyvinyl butyral resin, phosphoric acid and zinc tetroxy chromate necessitate the use of two-pack systems, are inconvenient to apply and are incompatible with some top coats.
As mentioned above, factory application of chromate coatings by the process described in British Patent Specification No. 1,234,181 gives satisfactory coatings on steel strip and other relatively flat surfaces but does not give satisfactory coating formation on highly convex surfaces and on external structures. For instance although it is possible to apply a satisfactory coating on the relatively flat surfaces of a pipeline by the process described in British Patent Specification No. 1,234,181 it is difficult or impossible to achieve a satisfactory coating on the ends of the pipe or on shaped pipe couplings and it is difficult to repair the coating if it is damaged, for instance after the pipe has been fixed in position.
It has been our object to devise a solution for forming satisfactory chromate coatings on surfaces that can be the surfaces of external structures and that can be highly convex or that can be pipe couplings or damaged parts of pipelines otherwise treated by a process as described in British Patent Specification No. 1,234,181.